In recent years, the demands for low-fuel consumption of automobiles are becoming severer in connection with social requests for energy conservation. In order to comply with such demands, as for tire performance, a further decrease in rolling resistance of tires has been desired. Techniques of optimizing tire structures have also been studied as techniques for decreasing the rolling resistance of tires. However, the most general technique employed is using a material having lower heat buildup as the rubber composition.
In order to obtain such a rubber composition having low heat buildup, there have heretofore been promoted many technical developments of modified rubbers for rubber compositions using silica or carbon black as a filler. Of these, a process of modifying polymerization active ends of conjugated diene polymers obtained by anionic polymerization using an organolithium compound with an alkoxysilane derivative having a functional group which interacts with the filler has particularly been proposed as an effective process.
However, many of these are applied to polymers in which the living properties of the polymer ends can be easily secured, and only a few are applied to improve the modification of cis-1,4-polybutadiene, which is particularly important in tire side wall rubber, tire tread rubber, and the like. Furthermore, the modifying effect in rubber compositions comprising silica or carbon black is not necessarily sufficient. In particular, the modifying effect of rubber in which carbon black has been incorporated is rarely achieved for cis-1,4-polybutadiene.
In order to overcome the above-mentioned deficiencies, a process in which an end-modified conjugated diene polymer is obtained by reacting an active end of a conjugated diene polymer having a high cis content obtained by using a rare earth catalyst with a functional group-containing alkoxysilane derivative which interacts with a filler, and a method of adding a condensation accelerator at the time of alkoxysilane modification (refer to Patent Documents 1 and 2) have been suggested. However, more advanced performance of resulting modified polymer is desired.
Patent Document 1: WO 03/046020 A1
Patent Document 2: JP-A-2005-8870